Fixing stencil buffer functionality in WebXR

WebXR is using so called 'opaque' framebuffer, i.e. all attachments are made on GL level and not visible by WebGL. When 'enable(gl.STENICL_TEST)' is called, the on WebGL context checks the WebGLFramebuffer object for presence of the STENCIL attachment and fails (because the opaque framebuffer doesn't have those).
I am adding an additional state to WebGLFramebuffer to track if the opaque framebuffer has the stencil buffer or not. Note, I can't check if the actual GL framebuffer object has the corresponding attachment, since the framebuffer might not be bound (yet).

The repro case: https://artyom17.github.io/webxr-samples/tests/stencil-cube-sea.html
In 2D mode you'll see that the green triangle has a cutout, while in VR mode it doesn't. This is because gl.enable(gl.STENCIL_TEST) is failing.

Bug: 1057571
Change-Id: I8b94a60de8c14ac7e435f17e57523ec430f22fb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084067Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Artem Bolgar <artyom@fb.com>
Cr-Commit-Position: refs/heads/master@{#747074}

third_party/blink/renderer/modules/webgl/webgl_framebuffer.cc

@@ -183,9 +183,12 @@ void WebGLTextureAttachment::Unattach(gpu::gles2::GLES2Interface* gl,
 
 WebGLFramebuffer::WebGLAttachment::WebGLAttachment() = default;
 
-WebGLFramebuffer* WebGLFramebuffer::CreateOpaque(
-    WebGLRenderingContextBase* ctx) {
-  return MakeGarbageCollected<WebGLFramebuffer>(ctx, true);
+WebGLFramebuffer* WebGLFramebuffer::CreateOpaque(WebGLRenderingContextBase* ctx,
+                                                 bool has_stencil) {
+  WebGLFramebuffer* const fb =
+      MakeGarbageCollected<WebGLFramebuffer>(ctx, true);
+  fb->SetOpaqueHasStencil(has_stencil);
+  return fb;
 }
 
 WebGLFramebuffer::WebGLFramebuffer(WebGLRenderingContextBase* ctx, bool opaque)
@@ -366,10 +369,14 @@ GLenum WebGLFramebuffer::CheckDepthStencilStatus(const char** reason) const {
 }
 
 bool WebGLFramebuffer::HasStencilBuffer() const {
-  WebGLAttachment* attachment = GetAttachment(GL_STENCIL_ATTACHMENT);
-  if (!attachment)
-    attachment = GetAttachment(GL_DEPTH_STENCIL_ATTACHMENT);
-  return attachment && attachment->Valid();
+  if (opaque_) {
+    return opaque_has_stencil_;
+  } else {
+    WebGLAttachment* attachment = GetAttachment(GL_STENCIL_ATTACHMENT);
+    if (!attachment)
+      attachment = GetAttachment(GL_DEPTH_STENCIL_ATTACHMENT);
+    return attachment && attachment->Valid();
+  }
 }
 
 void WebGLFramebuffer::DeleteObjectImpl(gpu::gles2::GLES2Interface* gl) {

third_party/blink/renderer/modules/webgl/webgl_framebuffer.h

@@ -71,7 +71,8 @@ class WebGLFramebuffer final : public WebGLContextObject {
   // An opaque framebuffer is one whose attachments are created and managed by
   // the browser and not inspectable or alterable via Javascript. This is
   // primarily used by the VRWebGLLayer interface.
-  static WebGLFramebuffer* CreateOpaque(WebGLRenderingContextBase*);
+  static WebGLFramebuffer* CreateOpaque(WebGLRenderingContextBase*,
+                                        bool has_stencil);
 
   GLuint Object() const { return object_; }
 
@@ -108,6 +109,9 @@ class WebGLFramebuffer final : public WebGLContextObject {
 
   bool Opaque() const { return opaque_; }
   void MarkOpaqueBufferComplete(bool complete) { opaque_complete_ = complete; }
+  void SetOpaqueHasStencil(bool has_stencil) {
+    opaque_has_stencil_ = has_stencil;
+  }
 
   // Wrapper for drawBuffersEXT/drawBuffersARB to work around a driver bug.
   void DrawBuffers(const Vector<GLenum>& bufs);
@@ -160,6 +164,7 @@ class WebGLFramebuffer final : public WebGLContextObject {
   bool web_gl1_depth_stencil_consistent_;
   bool contents_changed_ = false;
   const bool opaque_;
+  bool opaque_has_stencil_ = false;
   bool opaque_complete_ = false;
 
   Vector<GLenum> draw_buffers_;

third_party/blink/renderer/modules/xr/xr_webgl_layer.cc

@@ -122,7 +122,8 @@ XRWebGLLayer* XRWebGLLayer::Create(
                        framebuffers_size.Height() * framebuffer_scale);
 
   // Create an opaque WebGL Framebuffer
-  WebGLFramebuffer* framebuffer = WebGLFramebuffer::CreateOpaque(webgl_context);
+  WebGLFramebuffer* framebuffer =
+      WebGLFramebuffer::CreateOpaque(webgl_context, want_stencil_buffer);
 
   scoped_refptr<XRWebGLDrawingBuffer> drawing_buffer =
       XRWebGLDrawingBuffer::Create(webgl_context->GetDrawingBuffer(),